Rail electrification projects on some routes across the south of England are running up to four years later than planned, Network Rail (NR) has said.
The owner of Britain's rail track revealed the extent of the delays in a progress report on the upgrade of the G reat Western line between London and South Wales.
The project was due to be completed by 2018 but some routes which were originally scheduled to be ready before then will not be finished until 2020.
They includes electrification at Bristol Temple Meads station, which was initially planned to be done by December - some four years earlier.
Oxford electrification has been pushed back three years from the original aim of 2016 to 2019, while Newbury has a similar delay from 2015 to 2018.
An NR spokeswoman told the Press Association: "The Great Western main line to Cardiff - which forms the backbone of the electrification project and will carry the vast majority of services and passengers - will be electrified by 2018, the year originally envisaged.
"The scale and complexity of the electrification project, together with the other extensive upgrades we are carrying out, mean we will electrify other lines later than originally planned.
"Massive investment is taking place now and it will deliver faster, quieter and greener journeys for hundreds of thousands of passengers each year."
Great Western Railway, which operates trains on the route, issued a statement which read: " We will do everything we can to help Network Rail meet and where possible exceed their programme in the best possible way for customers.
"However, for now we will be working with Network Rail and the DfT (Department for Transport) to investigate alternative ways of delivering the full package of capacity and frequency improvements we promised in the new GWR franchise, despite the challenges of late electrification."
NR admitted in October that the cost of the project could reach £2.8 billion despite an estimate of just £ 874 million being given in January 2013.
Electrification schemes for the Midland Main Line and the TransPennine routes were restarted in September last year after being halted in June.
Ministers were accused of deceiving the public over their intentions for the rail network when the pause was announced just weeks after the general election.
The initial plan for the Midland Main Line north of Bedford was to reach Corby by December 2017, Derby and Nottingham by December 2019 and Sheffield by December 2020.
But a review by NR chairman Sir Peter Hendy published last year delayed the upgrade for Corby and Kettering until 2019, while the line to Leicester, Derby, Nottingham and Sheffield will not be electrified until 2023.
Electrification of the TransPennine line between Manchester and York is expected to provide capacity for six fast or semi-fast trains per hour and shave up to 15 minutes off current journey times between the cities.
The work was originally scheduled to finish by 2019 but is now not due to be completed until 2022.